I just wanted to bring this thread back to say that in my case, it is most likely burr on one of the teeth of the big ring. After reading this thread to find out why my bike has started having this problem all of a sudden, I went over and checked my front chain ring. There are a few teeth on the big ring, just behind the crank arm, that have some significant burrs on them . I don't have a file to deal with the problem right now, but I'll take it in or go buy a file.

Thanks.